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Old 19-12-2020, 08:16 PM
Zuts
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1.25 inch filters

Hi,

I have an ASI 2600 MC camera with an APS-C chip (23.5 x 17.5mm).

It is impossible atm to source a 2 inch filter drawer, but I can use a 1.25 inch filter if I get the adaptor. This would place the filter around 40mm from the chip, around the same place as if I used a filter drawer.

Is this filter too small and would it vignette, the chip is 23.5 mm and the filter is 31.75 mm, or would i be better of waiting for the filter draw and using my 2 inch filter.

Also currently the 2 inch filter is screwed into the end of my reducer so is further away than what I am proposing. I have been told that it is better to have the filter closer to the chip so this should be a better position?

Cheers
Paul

Last edited by Zuts; 20-12-2020 at 04:27 PM.
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Old 19-12-2020, 10:24 PM
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Camelopardalis (Dunk)
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Paul, it’s better to have the filter as close to the sensor _if_ their sizes aren’t too dissimilar. If the filter is much larger than the sensor, then it can be further away before causing an obstruction to light.

FWIW, 1.25” filters usually work out about 28.5mm clear aperture. How noticeable any obstruction is will depend on the focal ratio of your scope/reducer combo.

A 2” filter is substantially larger...
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Old 20-12-2020, 08:57 AM
jahnpahwa (JP)
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Hi Paul, I think your sensor is smaller than APS-C, at 17.7x13.4mm, and 1.25" filters are ok for that size. That said, 40mm seems a long way from the sensor so still may throw up an issue. Are there any secret threads inside spacers or adapters closer to the sensor that you could mount it in?
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Old 20-12-2020, 09:09 AM
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DiscoDuck (Paul)
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Paul, it depends also on your scope's focal ratio. There is a calculator here https://astronomy.tools/calculators/ccd_filter_size where you can plug in your numbers.
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Old 20-12-2020, 02:09 PM
Zuts
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Hi,

Thanks for all the replies, I have decided to wait for the 2 inch filter drawer. One of the reasons I was asking about distance was because I thought the further away the less chance of reflections and halos around stars, then I read it is the exact opposite?

Quote:
Originally Posted by Camelopardalis View Post
Paul, it’s better to have the filter as close to the sensor _if_ their sizes aren’t too dissimilar. If the filter is much larger than the sensor, then it can be further away before causing an obstruction to light.

FWIW, 1.25” filters usually work out about 28.5mm clear aperture. How noticeable any obstruction is will depend on the focal ratio of your scope/reducer combo.

A 2” filter is substantially larger...
Thanks Dunk, I hadn't considered the fact that the clear aperture of the filter is less than the actual aperture by over 10% in this case.

Quote:
Originally Posted by jahnpahwa View Post
Hi Paul, I think your sensor is smaller than APS-C, at 17.7x13.4mm, and 1.25" filters are ok for that size. That said, 40mm seems a long way from the sensor so still may throw up an issue. Are there any secret threads inside spacers or adapters closer to the sensor that you could mount it in?
Hi JP, the ASI 2600 MC is (23.5 x 17.5mm) with a diagonal of 29.3, maybe you were thinking of my ASI 1600 MC which I have replaced now. While I am waiting for the filter drawer I can use an M48/M42 adaptor but this will increase the spacing from reducer to chip by an extra 1mm beyond the recommended 55mm.

Quote:
Originally Posted by DiscoDuck View Post
Paul, it depends also on your scope's focal ratio. There is a calculator here https://astronomy.tools/calculators/ccd_filter_size where you can plug in your numbers.
Thanks for the link Paul. This convinced me to wait for the filter drawer and not get a 1.25 inch filter.


Cheers
Paul
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Old 20-12-2020, 04:02 PM
jahnpahwa (JP)
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Sorry Paul, yep, I was referring to the 1600mc
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